Scintillation Counters for XRay Astronomy

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In 1964, the first observation of hard X-rays from the Crab was performed by Clark [2] with a NaI(Tl) scintillation counter on a balloon payload. Subsequent balloon and satellite payloads used the well-type configuration. In balloon instruments, large (100 cm2) crystals were coupled to standard 5 in. phototubes. In the mid 1970s, the first phoswich detectors were built, first by the UCSD and MIT groups [8] followed soon by the MPE/AIT collaboration [9]. Because of this improved shielding technique, larger detectors with several hundred cm2 collecting area and low instrumental background could be developed. The MPE/AIT balloon payload HEXE

carried an array of 3 x 4 NaI(Tl)/CsI(Tl) phoswich detectors with a total collecting area of 2300 cm2. The series of Orbiting Solar Observatories OSO-3, OSO-5, OSO-7, and OSO-8, launched between 1967 and 1975, carried the first scintillation counters into earth orbit. Theses instruments with effective areas from 10 cm2 on OSO-3 to 65 cm2 on OSO-7 were mainly devoted to the exploration of the diffuse X- and Gamma-ray background [1]. The UCSD/MIT hard X-ray and low Gamma-ray experiment A4 on HEAO-1 (1977-1978) , which consisted of 7 phoswich detectors of three different types, performed the first high energy all sky survey. In this instrument, a combination of well-type and phoswich shielding was used resulting in a remarkably low instrumental background of (1-2)10-4 counts cm-2 s-1 keV-1 in the 220 cm2 low energy detector [4].

Ten years later a space qualified version of the MPE/AIT HEXE detector [10] was launched with the Kvant module and docked to the Soviet space station Mir. Figure 3.1 shows a cross section of this instrument. The Mir HEXE consisted of 4 phoswich detectors surrounded on five sides by a graded shield and covered on all six sides by Ne 110 plastic scintillator (5-mm thick at the top, 10-mm thick elsewhere). NaI(Tl) crystals (3.2-mm thick) were coupled to 50-mm thick CsI(Tl) crystals and sealed by a window made of an aluminium honeycomb structure covered on both sides by 0.1-mm thick aluminium foil. The field of view (FOV) was defined to 1.6° x 1.6° (FWHM) by two collimators made of hexagonally shaped tungsten tubes. The collimators could separately be tilted by 2.5° to provide simultaneous source and background measurements. The tungsten collimators reduced the geometric area by less than 10%. For the gain control of the phoswich detectors there were four calibration sources mounted inside the passive shield. Am241 was tungsten collimator 1.6°1.6°

4 phoswich detectors 3.2 mm Nal(TI) 50 mm Csl(TI)

phototube EMI D 388 NA

tungsten collimator 1.6°1.6°

4 phoswich detectors 3.2 mm Nal(TI) 50 mm Csl(TI)

phototube EMI D 388 NA

graded shield 3 mm lead 2 mm tin 1 mm brass

Am 241 cal source

NE 110 plastic AC 4 sides and bottom: 10 mm top: 5mm

Fig. 3.1 Cross section of the Mir HEXE phoswich detector assembly

graded shield 3 mm lead 2 mm tin 1 mm brass

Am 241 cal source

NE 110 plastic AC 4 sides and bottom: 10 mm top: 5mm

Fig. 3.1 Cross section of the Mir HEXE phoswich detector assembly embedded in plastic scintillators providing tagging of the calibration photons by the light pulses from the simultaneous alpha particles with better than 95% efficiency. Although the orbit of the Mir station (i = 56°) regularly passed through the South Atlantic Anomaly and through the polar radiation belts, the detector background was quite low on average, see Table 3.2. The quadratic shape of the phoswich crystals allowed for a dense packing of the detector assembly, providing mutual shielding. However, the transition from the quadratic crystal to the circular phototube was not ideal in terms of light collection, resulting in a reduced energy resolution of only 25% at 60keV. Until January 1993, about 1 800 observation sessions with a total observing time of 1.9106 s were performed with the Mir HEXE detector.

About 9 years after Mir Kvant, two further hard X-ray phoswich detectors were launched into low earth orbit, first the high energy X-ray timing experiment (HEXTE) on RXTE (December 1995) [11] and then the Phoswich detection system (PDS) on BeppoSAX (April 1996) [3]. Similar to Mir HEXE, both instruments use rocking collimators for continuous background monitoring. Both detectors reach about 15% relative energy resolution at 60 keV. This was achieved by a careful shaping of the shielding crystal and in the case of RXTE by the use of circular detectors. In Table 3.2, we summarize the main properties of both instruments in comparison to Mir HEXE and the Suzaku HXD detector.

While in the past all balloon and satellite phoswich detectors used alkali halide crystals, the hard X-ray detector (HXD) on board of the Japanese X-ray astronomy satellite Suzaku (launched July 2005 into an 31° inclined low earth orbit) utilizes an array of 5-mm thick GSO crystals as the main detector for the 40-600 keV energy band [7]. Low energy (10-70 keV) photons are measured by 2-mm thick silicon PIN type diodes in front of the GSO crystals with very good energy resolution of

Table 3.2 Hard X-ray phoswich detector instruments

Instrument

Mir HEXE

RXTE HEXTE

SAX PDS

Suzaku HXD

Type

Energy band Rel. energy res. at 60 keV Net open area FOV (FWHM) Background3 in units of 10~4 cts cm~2 keV

3.2mm NaI(Tl) 3mmNaI(Tl) 3mmNaI(Tl) 2mm Si PIN 50 mm CsI(Tl) 57mmCsI(Na) 50mmCsI(Na) 5 mm GSO

15-200 keV 25%

15-250keV 15.4%

15-300 keV 15%

4c 2

10-600keV 30%

a Typical background values (for Mir HEXE and RXTE HEXTE for part of orbit with high magnetic rigidity) are quoted for three energy bands: 20-40, 40-80, and 80-200 keV (top to bottom)

b R. Rothschild, priv. comm.

c From BeppoSAX Observer's Handbook d PIN and GSO background 220 days after launch [6]

3 keV (FWHM). The HXD is composed of a matrix of 4 x 4 well-type units surrounded at four sides by active BGO shielding units. Each well unit contains four stacks of a PIN diode and a GSO crystal deep inside an active collimator made of 3-mm thick BGO plates for the four side walls and a central cross. The 320-mm long collimator plates and the four GSO crystals are optically glued to a 60-mm thick BGO shielding crystal, coupled to one phototube. The FOV of the active collimator is 4.5° x 4.5° (FWHM). For photons below 100 keV, it is further confined to 0.57° x 0.57 ° (FWHM) by fine collimators made of 50 |im phosphor bronze. Because of the combination of well-type and phoswich-type shielding, the background between 20 and 50keV is much lower than in previously flown missions, while above 50 keV the background is similar to standard phoswich detectors [6].

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